Purpose: Isoluminant Hermann grids (HG) are typically not effective inducers of chromatic Hermann grid illusions (HGI). Oehler and Spillmann (1981) devised an isoluminant HG that resulted in strong chromatic HGIs that appeared to be induced by L and M cones but not S cones. We wished to determine the influence of the S cones on the chromatic HGI.

Method: 63 Isoluminant Hermann grids varied systematically in stimulation of the L, M and S cones. They were defined by three components: continuous neutral-colored vertical bars, horizontal line segments (21 varieties), and background squares (purple, neutral or yellow-green). The background squares differed from each other only in S cone excitation. 6 subjects rated the magnitude of the illusion. As a control, a series of achromatic grids were interspersed with the chromatic grids.

Results: For the purple background, there was a significant (P[[lt]].0001) increase in the strength of the illusion with a decrease in S cone stimulation of the horizontal line segments. This effect was greatest when the L/M cone ratio for the horizontal line segment was largest. For the yellow-green background, there was a significant (P[[lt]].0001) increase in the strength of the illusion with an increase in S cone stimulation of the horizontal line segments. This effect was smallest when the L/M cone excitation ratio for the horizontal line segment was largest. There was little or no illusion seen with a neutral background.

Conclusion: S cones participate in producing the chromatic Hermann grid illusion. The illusion varied for the different background squares, even though they were identical in L/M cone excitation and differed only in S cone stimulation. Under some conditions, increasing the cone excitation difference between horizontal line segments and the background resulted in weaker illusions, suggesting a suppressive effect of color noted in our previous work (Comerford et al VSS2005).